CLI improvements

[ehllie]

Ideas for changes so far

Overhauling the cli by splitting the functionality into 3 sub-commands:

• check: Equivalent of ante --check and has options for printing out the result of each step before codegen.
• build: Equivalent of ante --build and has the options for configuring the backend and optimisation level.
• run: The current default ante, has the same options as build, but also one to delete the binary afterwards.

Additionally, a new functionality inside the sub-command completions which just allows for automatically generating code completion scripts with clap_complete

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[jfecher]

Adding support for command line completion would be great, is there any way we can do it without changing the default of ante run ... though?

[ehllie]

I went for separating the cli into sub-commands, to clearly differentiate between which options are compatible with one another, but it should also be possible to achieve with argument relations

[ehllie]

https://user-images.githubusercontent.com/20847625/194134698-357cc935-2188-4018-8a27-ccb184a45494.mp4

This is what I have so far

[ehllie]

I can alternatively try doing something like this:

• Group build, delete-binary, show-hir, show-ir, backend and opt_level flags and options into a codegen args struct
• Make build and delete-binary flags mutually exclusive in that struct
• Make check flag mutually exclusive with the codegen args fields
• Make a print-completion option that's mutually exclusive with all other options

That should preserve the current functionality, add some extra check and allow for generating syntax completion.

[jfecher]

That sounds better. As far as prioritization goes, I think the most important aspects are (in order):

1. Getting some command line completion working
2. Maintaining the current command line syntax, or keeping it flexible for the future. E.g. a large change that will eventually be necessary is forwarding all arguments after the filename when running a file to the argv of that program. So that you could invoke it via ante myecho.an this is echoed.
3. Ease of using the clap/cli API for the developer, brevity of code etc. If it requires too much code it may not be worth it for such a simple thing.
4. Least important to me is preventing mutually conflicting options like check and run. I think most of these should be fairly obvious to users that they are conflicting, but an explicit error would always be nice.

[ehllie]

This PR is a little more conservative now.

• Renamed args.rs to cli.rs and the arg struct to Cli to avoid conflicts with clap::Args
• Code completion scripts can be generated using ante --shell-completion <shell>
• Cli should issue warnings in case the user tries to use more than one of lex parse check or build at the same time
• Updated the nix derivation to install code completion for ante